[infra] Fix flutter-frontend

The third_party directory was removed in https://github.com/flutter/buildroot/commit/d45d8382665c923bdc2f8ab4e49f0deef84dd173

Change-Id: Ib487b801d7446a151b6a1f231e3761ef6158a73f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/223801
Reviewed-by: William Hesse <whesse@google.com>
diff --git a/tools/bots/flutter/compile_flutter.sh b/tools/bots/flutter/compile_flutter.sh
index 511855c..35d271f 100755
--- a/tools/bots/flutter/compile_flutter.sh
+++ b/tools/bots/flutter/compile_flutter.sh
@@ -73,6 +73,7 @@
 pushd src
 git clone --single-branch --depth=1 -vv \
     https://dart.googlesource.com/external/github.com/flutter/engine flutter
+mkdir -p third_party
 pushd third_party
 ln -s $checkout dart
 popd  # third_party